A chipper, or chipping machine, can be considered as consisting of three essential pieces or parts within a robust, rigid framework. A spout serves to guide debarked logs that are to be formed into wood chips for subsequent pulpmaking into a knife radially mounted on a rotating disk, which slices through the wood at the angle of the spout. As the knife bears against the log, the opposite surface of the log is pressed against an anvil. The knife acts as a wedge, creating a shearing force in the grain of the wood. As the shearing force builds to a critical point, the stress parallel to the grain of the wood causes chips to be formed, as will be illustrated herein.
The anvil, together with the spout, is intended to carry the piece at the correct location and orientation for the chip forming action of the knives and disk. Wear of the infeed spout causes misalignment of solid wood at the anvil/knife contact point, producing inconsistent chip size. The anvil edge needs to be sharp. If the knives and the anvil act together in a close scissor-like configuration, the log or piece can "ride easy" in the spout. Otherwise, especially if the log or piece is short or small in cross section, it can be tipped or thrown out of its correct position, resulting in bad chips. Dull knives aggravate this effect.
It will be understood then, that it is necessary to maintain the anvil of a chipper in prime condition, if efficient operation of the chipper is desired. To this end, the American Pulpwood Association recommends servicing the anvil of a chipper once a week. Servicing of an anvil will entail replacement, renewal (i.e., edge grinding, if enough stock remains) or turning, if an unworn edge of the anvil is available. It will be appreciated that replacement of an anvil, which is a heavy piece of very high grade tooled steel can be quite expensive, and renewal of anvils edges, while less expensive in material consumption, requires a machine to be shut down while the edge is reground by a skilled wright. Turning an anvil is a temporary solution only, since an anvil provide four edges at most. Moreover, to be turned, the anvil must be completely removed from its mounting.